The invention concerns a furnace, especially a fluidized furnace.
In one known furnace (EP Patent 103 613) that is provided with a circulating fluidized bed, the solids are separated from the flue gas by spinning-particle collectors, collected in a hopper, and returned to the fluidized bed. The solid matter is not significantly cooled before returning to the bed because the hopper has no flue surfaces, with the exception of its walls, and because the solid matter is fluidized with just enough air to ensure uniform conveyance.
In another known circulation-based fluidized furnace (German Patent 2 624 302), the solid matter is kept in circulation, suspended at a high density in the flue gas and at a constant system temperature, by a system comprising a fludized-bed reactor, a cyclone precipitator, and a recirculation line. Some of the solid matter is removed from circulation and cooled in a separate flowing-bed cooler that is provided with flue surfaces and that has air flowing through it, with at least some of it being returned to the fluidized furnace.
Also known (BWK 40 [1988], pp. 273-76) is a furnace with a fluidized bed and boosted solid-matter circulation and with nested flue surfaces for cooling the flue gas and fly ash in the space above the bed.
The stagnant fluidized bed in another known furnace (VGB Kraftwerkstechnik 67, 8 [1987], pp. 571-77) has flue surfaces immersed in it. This furnace is operated under pressure, and the fluidized combustion chamber is accommodated in a pressurized vessel. The flue gas leaving the combustion chamber is forwarded to a gas turbine.
In the known fluidized furnaces a current of heat is released in the fluidized layer of solids and transmitted to the flue surfaces or along with the flue gas to the combustion chamber. The flue surfaces in furnaces with a stagnant fluidized bed are of the immersion type. Since the cross-section of the fluidized bed cannot be expanded without limits for reasons of design and process technology, the rate of fluidization will be relatively high at a given output. The immersion-type flue surfaces inside the bed are accordingly subjected to severe friction, which can wear down the material.
In fluidized beds with a boosted solids circulation and in circulating fluidized beds there is no need for immersion-type flue surfaces in the fluidized combustion chamber. The combustion chamber is instead provided with flue surfaces in the form of walls or partitions for the flue gas, heavily charged with particles, to flow over. The amount of gas that occurs will be dictated by the prescribed output. Since the flow cross-section cannot for reasons of design be as large as desired, the gas will flow at at least a minimum speed and, heavily laden with particles, will erode the surface of the walls and partitions.
When air is introduced discontinuously to extensively decrease the emissions of NO.sub.x, the reducing conditions that will exist to some extent in the reactor will also corrode the flue surfaces, an attack that can be repulsed only by using high-quality materials.
Furnaces that are operated under pressure and have a stagnant fluidized bed are controlled by lowering the level of the bed as the load decreases. When the bed level is low, some of the immersion-type heating surfaces will extend out of it, and less heat will be diverted from it. The temperature, however, will decrease simultaneously, and gas will be supplied to the downstream turbine at a temperature that depends on the load.